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Not Applicable
The present invention relates to optical microcavity resonators, and in particular to highly efficient, low-loss optical microcavity resonators having relatively high quality factors (Q""s).
During the past few years, a substantial amount of research has been performed in the field of optical microcavity physics, in order to develop high cavity-Q optical microcavity resonators. In general, resonant cavities that can store and recirculate electromagnetic energy at optical frequencies have many useful applications, including high-precision spectroscopy, signal processing, sensing, and filtering. Many difficulties present themselves when conventional planar technology, i.e. etching, is used in order to fabricate high quality optical resonators, because the surfaces must show deviations of less than about a few nanometers. Optical microsphere resonators, on the other hand, can have quality factors that are several orders of magnitude better than typical surface etched resonators, because these microcavities can be shaped by natural surface tension forces during a liquid state fabrication. The result is a clean, smooth silica surface with low optical loss and negligible scattering. These microcavities are inexpensive, simple to fabricate, and are compatible with integrated optics.
Optical microcavity resonators have quality factors (Qs) that are higher by several orders of magnitude, as compared to other electromagnetic devices. Measured Qs as large at 1010 have been reported, whereas commercially available devices typically have Qs ranging from about 105 to about 107. The high-Q resonances encountered in these microcavities are due to optical whispering-gallery-modes (WGM) that are supported within the microcavities.
As a result of their small size and high cavity Q, interest has recently grown in potential applications of microcavities to fields such as electro-optics, microlaser development, measurement science, and spectroscopy. By making use of these high Q values, microspheric cavities have the potential to provide unprecedented performance in numerous applications. For example, these microspheric cavities may be useful in applications that call for ultra-narrow linewidths, long energy decay times, large energy densities, and fine sensing of environmental changes, to cite just a few examples.
In order for the potential of microcavity-based devices to be realized, it is necessary to couple light selectively and efficiently into the microspheres. Since the ultra-high Q values of microcavities are the result of energy that is tightly bound inside the cavity, optical energy must be coupled in and out of the high Q cavities, without negatively affecting the Q. Further, the stable integration of the microcavities with the input and output light coupling media should be achieved. Also, controlling the excitation of resonant modes within these microcavities is necessary for proper device performance, but presents a challenge for conventional waveguides.
In general, the desirable characteristics of a microcavity coupler include: 1) efficient WGM excitation; 2) easy alignment of the microcavity with respect to a coupler; 3) clearly defined ports; 4) a robust and integrable structure; and 5) a consistent and inexpensive fabrication process. One of the most efficient prior art methods incorporates phase-matched evanescent wave coupling. One commonly used approach for phase-matched evanescent wave coupling is to polish down the cladding of an optical fiber, until the evanescent field is locally exposed. Other techniques have been used in the prior art for coupling light into the microspheres, for example the prism coupler, and the tapered fiber coupler. For the tapered coupler, a tapered fiber is formed, i.e. a narrow waist is formed on a fiber by heating and gradual stretching.
While the above-mentioned techniques provide efficient coupling, these approaches suffer from a number of drawbacks. For example, most currently existing techniques for the excitation of whispering-gallery-mode (WGM) resonances in optical microcavities are not easily scalable for mass production. Also, the existing techniques are not robust or versatile enough for desired measurement environments. The fabrication of both the exposed fiber and the tapered fiber is nontrivial and intricate, and the resulting couplers are rather fragile. In particular, the tapered fiber coupler requires delicately drawn fibers, less than 5 micrometers in diameter and suspended in air. Also, the prism coupler does not provide guided wave control. Further, the prism coupler uses bulk components, and is therefore less desirable for applications that call for robustness.
Typically, good overall performance is gained by accessing the evanescent field in a waveguide. Also, only waveguide structures provide easy alignment and discrete, clearly defined ports. Leakage from the sphere WGMs onto the fiber cladding modes lowers the coupling efficiency, however. High-Q microcavities are typically composed of high-purity silica, a material whose low refractive index value is commonly used for the cladding of planar fiberoptic waveguides. As a result, a silica sphere coupled to a conventional surface waveguide will lose most of its energy to substrate and cladding radiation. This loss spoils the Q, and reduces the device efficiency. Because of cavity and waveguide mode leakage into the substrate and into the modes within the fiber cladding, power extraction from the input optical radiation is inefficient.
There is a need for a robust and efficient system for coupling light into high Q optical microcavities, so that the high Q values can be fully utilized.
A method and system is presented for efficiently and robustly coupling optical radiation into an optical microcavity resonator so as to excite resonance modes within the microcavity. In particular, high-Q optical microspheric cavity resonators are evanescently coupled to an optical waveguide chip that has a SPARROW (Stripline Pedestal Anti-Resonant Reflective Optical Waveguide) structure. When the frequency of the light propagating along the waveguide matches a resonant whispering gallery mode (WGM) of the microspheric cavity, light is coupled into the microsphere. Coupling efficiencies of over 98% may be attained.
The present invention features a low-loss, high-Q optical resonator system. The optical resonator system includes a substrate, an optical waveguide, and an optical microcavity, all integrated into a single structure. The substrate is preferably substantially planar, and may be made of silicon, by way of example. The optical waveguide includes a multi-layer dielectric stack disposed on the substrate. The dielectric stack includes alternating high and low refractive index dielectric layers. A waveguide core is disposed on top of the dielectric stack. The waveguide core has an input end and an output end. The waveguide core is adapted for transmitting optical radiation incident on the input end to the output end.
The optical microcavity is constructed and arranged so as to optically interact with optical radiation propagating through the optical waveguide. The optical microcavity may be a microdisk, a microsphere, or a microring, by way of example. In one embodiment, the optical microcavity may be fabricated by melting a tip of a silica optical fiber or wire. The optical microcavity may be substantially spherical in shape, and characterized by a diameter of about 50 micrometers to about 500 micrometers.
Because of the alternating high- and low- index dielectric layers, the reflectivity of the dielectric stack is very high. In particular, the reflectivity of the dielectric stack is high enough to isolate the optical modes in the microcavity and in the waveguide core from the substrate. The maximum distance between the optical microcavity and the optical waveguide is sufficiently small so as to allow evanescent coupling of light from the waveguide into the microcavity, namely the maximum distance is of the order of the wavelength of the light incident upon the waveguide.